Chapter 5 Input/Output 5.1 Principles of I/O hardware 5.2 Principles of I/O software 5.3 I/O software layers 5.4 Disks 5.5 Clocks 5.6-8 Terminals

Principles of I/O Hardware Some typical device, network, and data base rates

Device Controllers I/O devices have components: mechanical component electronic component The electronic component is the device controller may be able to handle multiple devices Controller's tasks convert serial bit stream to block of bytes perform error correction as necessary communicate with CPU

Memory-Mapped I/O (1) Separate I/O and memory space Memory-mapped I/O Hybrid

Memory-Mapped I/O (2) (a) A single-bus architecture (b) A dual-bus memory architecture

Direct Memory Access (DMA) Operation of a DMA transfer

Interrupts Revisited How interrupts happens. Connections between devices and interrupt controller actually use interrupt lines on the bus rather than dedicated wires

Principles of I/O Software Goals of I/O Software (1) Device independence programs can access any I/O device without specifying device in advance (floppy, hard drive, or CD-ROM) Uniform naming name of a file or device a string or an integer not depending on which machine Error handling handle as close to the hardware as possible

Goals of I/O Software (2) Synchronous vs. asynchronous transfers blocked transfers vs. interrupt-driven Buffering data coming off a device cannot be stored in final destination Sharable vs. dedicated devices disks are sharable tape drives would not be

Steps in printing a string Programmed I/O (1) Steps in printing a string

Writing a string to the printer using programmed I/O --- busy waiting

Interrupt-Driven I/O Writing a string to the printer using interrupt-driven I/O Code executed when print system call is made Interrupt service procedure

I/O Using DMA Printing a string using DMA code executed when the print system call is made interrupt service procedure

Layers of the I/O Software System I/O Software Layers Layers of the I/O Software System

Interrupt Handlers (1) Interrupt handlers are best hidden have driver starting an I/O operation block until interrupt notifies of completion Interrupt procedure does its task then unblocks driver that started it

Interrupt Handlers (2) Steps must be performed in software after interrupt completed Save registers not already saved by interrupt hardware Set up context for interrupt service procedure Set up stack for interrupt service procedure Acknowledge interrupt controller, reenable interrupts Copy registers from where saved Run service procedure Set up MMU context for process to run next Load new process' registers Start running the new process

Device Drivers Logical position of device drivers is shown here Communications between drivers and device controllers goes over the bus

Tasks of Device Drivers Accept abstract requests Check input parameters Translate from abstract to concrete Check if device is in use Issue commands to controller (Block) Check errors Return (error) to caller

Device-Independent I/O Software (1) Uniform interfacing for device drivers Buffering Error reporting Allocating and releasing dedicate devices Providing a device-independent block size Functions of the device-independent I/O software

Device-Independent I/O Software (2) (a) Without a standard driver interface (b) With a standard driver interface

Device-Independent I/O Software (3) (a) Unbuffered input (b) Buffering in user space (c) Buffering in the kernel followed by copying to user space (d) Double buffering in the kernel

Device-Independent I/O Software (4) Networking may involve many copies

User-Space I/O Software Layers of the I/O system and the main functions of each layer

Disks Disk Hardware (1) Disk parameters for the original IBM PC floppy disk and a Western Digital WD 18300 hard disk

Disk Hardware (2) Physical geometry of a disk with two zones A possible virtual geometry for this disk

Disk Hardware (3) Raid levels 0 through 2 Backup and parity drives are shaded

Disk Hardware (4) Raid levels 3 through 5 Backup and parity drives are shaded

Disk Hardware (5) Recording structure of a CD or CD-ROM

Logical data layout on a CD-ROM Disk Hardware (6) Logical data layout on a CD-ROM

Disk Hardware (7) Cross section of a CD-R disk and laser not to scale Silver CD-ROM has similar structure without dye layer with pitted aluminum layer instead of gold

A double sided, dual layer DVD disk Disk Hardware (8) A double sided, dual layer DVD disk

Disk Formatting (1) A disk sector

An illustration of cylinder skew Disk Formatting (2) An illustration of cylinder skew

Disk Formatting (3) No interleaving Single interleaving Double interleaving

Disk Arm Scheduling Algorithms (1) Time required to read or write a disk block determined by 3 factors Seek time Rotational delay Actual transfer time Seek time dominates Error checking is done by controllers

Disk Arm Scheduling Algorithms (2) Initial position Pending requests Shortest Seek First (SSF) disk scheduling algorithm

Disk Arm Scheduling Algorithms (3) The elevator algorithm for scheduling disk requests

Error Handling A disk track with a bad sector Substituting a spare for the bad sector Shifting all the sectors to bypass the bad one

Analysis of the influence of crashes on stable writes Stable Storage Analysis of the influence of crashes on stable writes

Clocks Clock Hardware A programmable clock

Clock Software (1) Maintaining the time of day Checking process allocation times Accounting for CPU usage Issuing warnings Providing watchdog timers

Three ways to maintain the time of day Clock Software (2) Three ways to maintain the time of day

Simulating multiple timers with a single clock Clock Software (3) Simulating multiple timers with a single clock

Soft Timers A second clock available for timer interrupts specified by applications no problems if interrupt frequency is low Soft timers avoid interrupts kernel checks for soft timer expiration before it exits to user mode how well this works depends on rate of kernel entries